@Alex Warlorn:
It's called The Masquerade or a Wainscot. This particular trope doesn't just rub me the wrong way; I flat out hate it. You know why most writers employ the trope? To engineer feelings of empathy and familiarity between the reader and the characters. It's hard to identify with the characters in science fiction or fantasy stories, particularly when set in a secondary world, as they don't live in "our" world. But if the protagonists happen upon some hidden magical world beneath our very noses it gives the author all the freedom of the fantasy genre without having to work to make the characters relatable. It's more egregious when the excuse for The Masquerade is something flimsy like "mankind isn't ready for the truth" or "it's magic; people will never notice it." That's how you can tell it's merely a cheap literary device. Harry Potter and Perry Jackson are particularly bad offenders.

Or 'he' already is one and was using the appearance of her former self as bait since Neko Cop would make an effective sister to begin the conversion of humanity.

Since Neko Cop called it dangerous, it's safe to say this takes place before the Legion abandoned its extremist methods. (Which would be a nice change of pace since we keep HEARING about how the legion used to be, and are now just poor and misunderstood.)